In the formation of a semiconductor chip, integrated circuit devices, such as transistors, are first formed at the surface of a semiconductor substrate in the semiconductor chip. Interconnect structures are then formed over the integrated circuit devices. Bumps are formed on the surface of the semiconductor chip so that the integrated circuit devices can be accessed.
In a typical bump formation process, an under-bump metallurgy (UBM) is formed, followed by the formation of a bump on the UBM. The UBM formation may include forming a copper seed layer and forming and patterning a mask on the copper seed layer so that a portion of the copper seed layer is exposed through an opening in the mask. A plating step is then performed to plate a thick copper layer on the exposed portion of the copper seed layer. Subsequently, additional layers, such as a nickel layer and a solder layer may be plated successively. Conventionally, between each of the plating processes, a quick dump rinse (QDR) is performed using de-ionized (DI) water to clean the surfaces of the plated layers.
It was found that the interfaces between the plated layers were rough, with voids formed therein. As a consequence, the electro-migration (EM) performance, and hence the reliability, of the resulting bumps is degraded.